In mobile communication networks, mobile handsets communicate with the fixed network via wireless communication links or channels. The network entity that manages wireless communication channels for a particular geographic area is referred to as a base station. Mobile communication networks typically include many base stations—one for each geographic area or cell.
When a mobile subscriber moves from one location to another location during a call, it may be necessary to hand the call off from one base station to another base station or between antennas within the same base station in order to maintain call quality. Handoff is a process by which a call, or active communication link, is transferred from one communication channel to another. There are three types of handoff that occur in mobile communication systems—hard handoff, soft handoff, and softer handoff. A hard handoff is the transfer of one communication channel to another communication channel that has not been previously established. A soft handoff is a conversion from one communication channel to another previously established communication channel where the two communication channels are with different base stations. A softer handoff refers to a transfer from one communication channel to another previously established communication channel where the two communication channels are with the same base station. A softer handoff may occur when a mobile station moves between areas served by different antennas of a base station.
Although the present invention can be applied to all three types of handoffs, a discussion of soft handoffs illustrates problems common to all three handoff methods. Soft handoff is initiated and ended based on the strength of a pilot signal received by a mobile station. A pilot signal is a signal transmitted from a base station to a mobile station via a control channel. Mobile stations measure the signal strength of the pilot signal to determine whether a handoff is necessary. For example, if the signal strength of a pilot signal from one base station exceeds a threshold value, the base station may determine that handoff of an active communication channel from the current base station to a new base station is required.
The process of soft handoff involves finding an available communication channel on the neighboring base station towards which the mobile station is moving in order to establish an active communication link. The available channels on a base station may be divided into two groups—free channels and soft guard channels. Free channels are communication channels available to mobile stations requesting a communication channel to establish a communication link. Soft guard channels are exclusively reserved for those mobile stations requesting a communication link in the handoff area. The reason for having soft guard channels reserved exclusively for handoffs is to reduce the likelihood of calls being dropped when a mobile station moves from one geographic area to another. In other words, the network designers decided to reserve a portion of base station resources exclusively for handoff calls rather than new calls because they decided it was better to refuse to admit new calls than to drop calls in progress. If a free or soft guard channel is not available on the neighboring base station, the communication link between the mobile station and the communication system may be dropped once the mobile station is out of the coverage area of the base station with which it is actively communicating.
Mobile stations continuously measure pilot signal strength with multiple base stations and classify the base stations in one of three sets based on signal strength: a neighborhood set; a candidate set; and an active set. The neighborhood set contains communication channels that are likely candidates for handoff but not currently in the active set or candidate set. The candidate set contains base stations having at least one available communication channel that could successfully establish a communication link between the base station and mobile station. The active set contains the base stations with which the mobile station has an active communication link on which speech frames or other data frames are transmitted. When the received pilot signal strength exceeds threshold TADD, the detected pilot signal is transferred from the neighborhood set to the candidate set. The threshold TADD is a predefined pilot signal strength value considered acceptable for establishing a communication link. If a base station receives a message indicating handoff completion from the mobile station, the associated base station is transferred from the candidate set to the active set. If the pilot signal strength received by the mobile station from the base station of the current cell is less than a predefined threshold TDROP, the base station is transferred from the active set to the neighborhood set and a message indicating handoff completion is transmitted to the base station.
The coverage area of the overlap of cells in soft handoff is known as the handoff area. The handoff area is primarily controlled by the handoff thresholds, TADD and TDROP. The area of a cell not covered by a handoff area (i.e., does not overlap the geographic area covered by another cell) is known as the normal area. The soft handoff process is defined in the Telecommunications Industry Association (TIA) and Electronic Industries Alliance (EIA) IS-95 publication, “Mobile Station-Base Station Compatibility Standard for Dual-mode Wideband Spread Spectrum Cellular System,” which is incorporated herein by reference. In the soft handoff process defined in US IS-95, there can be two or more base stations in the active set. When the signal strength of one of the pilot signals of an active set base station is less than TDROP, the corresponding call will leave the handoff area for the normal area after a short duration of time.
Although the capacity of CDMA systems is interference limited in nature, channel shortages may occur and utilization efficiency of traffic channels decreases because mobile stations may use several channels simultaneously when located in the handoff area. For example, while in the handoff area, a mobile station requests a soft handoff to a neighboring base station whenever the received pilot signal strength from the neighbor base station exceeds the predetermined threshold, although the mobile station is not actually approaching the neighboring base station. In this case, there exists unnecessary soft handoff calls requested by a mobile station moving in a direction away from the neighboring base station or a mobile station that is stationary. Typically, in urban CDMA cellular systems, the handoff area occupies approximately 30–50% of the cell area. Therefore, in a handoff process in which mobile stations in the handoff area unnecessarily occupy two or more communication channels, the number of available communication channels is reduced and the probability that a call will be dropped is increased. Therefore, a need exists for an improved communication channel allocation method that discriminates between calls in the handoff area. Furthermore, a process for allocating channels for handoff is desired which does not significantly increase computation complexity.